You can change this string in file en_US.po (default one) which is in the languages folder, just need to change the text .You will need to use the editor like POEdit to do this.
http://www.poedit.net/ -Poedit is the popular free editor for creating .po and the corresponding binary .mo files.
You may need to do an update in POEdit to see all the strings.
Better to change strings this way than changing the original code.
Or you can modify this in original file single-trainer.php at line no:60.